1. Field of the Invention
The present invention relates to mixed voltage drive systems for floating substrate technology environments, and more specifically for input/output ("I/O") buffer circuits.
2. Description of the Related Art
Input and output devices in data systems typically pass signals between each other. Oftentimes these devices have different voltage and current requirements and, therefore, their operation may be impaired if they are connected directly to one another. Direct connections may also cause signal miscommunication because the signal lines between devices can only carry the signals of one device at a time.
An I/O buffer is a circuit that, when enabled, passes digital signals between its device and one or more other devices, and, when disabled, isolates its device from the voltage and current loads of the other devices. Buffers may be selectively enabled and disabled and their corresponding devices may be granted or denied access to the signal lines interconnecting the devices. This allows for a protocol in which one device accesses the signal lines through its enabled set of buffers, while other devices are isolated by disabled buffers.
In a configuration having a first device and a second device such as a peripheral component, signals are often passed between each other. The two devices may operate at different voltages if they are directly connected one or both may be damaged or impaired. The two devices are protected, however, if they are coupled together by an I/O buffer circuit which isolates each device from the other's operating voltage and current.
A mixed voltage I/O buffer circuit performs all of the functions of the standard buffer described above and converts input signals of one voltage into logically equivalent output signals of another voltage. This is useful in passing signals between devices with distinct voltage thresholds. For example, a device may output signals at a maximum of 3.3 volts, while a peripheral component may require a minimum input voltage of 5.0 volts. Likewise, the peripheral component may output signals at a maximum of 5.0 volts, but the device may operate best with signals at 3.3 volts. A mixed voltage I/O buffer circuit placed between the two devices isolates each from the other's operating voltage and converts the 3.3 volts signal stream to a logically equivalent 5.0 volts signal stream and vice versa.
Conventional buffer circuits, however, suffer from several performance problems when used in floating substrate technology environments such as dynamic random access memories ("DRAMs"), for example. When mounted on a microchip, the negatively biased substrate of the DRAM preferably acts as a substrate for all other devices mounted on the microchip, including any I/O buffers.
If, however, a p-channel pull-up transistor was to be mounted on the microchip, the n-material substrate cannot be directly connected to the pull-up voltage as in the conventional circuit. This is because a direct connection between the negatively biased substrate and the positive pull-up voltage will result in a current flow into the DRAM substrate. If, however, the negatively biased substrate of the DRAM functions as the substrate of the pull-up transistor, a sufficiently positive voltage on a drain of the pull-up transistor will forward bias the PN junction between the drain and the substrate. The forward bias will breach the junction and create a current flow into the negatively biased substrate that would continue to increase to the extent that would lead to the destruction of the integrated circuit.
Another performance problem of a conventional buffer circuit is the amount of ground noise that may be generated by a sudden pull-down of the n-channel transistor when the transistor is placed in an ON state. Typically, the amount of noise produced is proportional to the rate at which the pull-down is performed. When the N-channel transistor is suddenly pulled down, there is a large amount of noise that is generated that, in turn, disrupts any process occurring with the DRAM. For example, a large amount of noise may disrupt a DRAM process when the transient current disturbs the negative bias on the substrate which may lead to voltage reversal and subsequent data destruction.
Therefore, there is a need for a system and a method for a mixed voltage drive system, including a buffer I/O system, for a floating substrate technology environment without leaking current into a negatively biased substrate and without producing excessive ground noise so that the I/O operation will not disrupt the operation of other portions of the integrated circuit.